On its website, VoteVets says it "works with all progressive allies representing labor, immigration, gay and lesbian rights, and environmentalists, when their issues coincide with the needs of troops and veterans."

The organization has opposed the Trump administration on a wide range of issues, although neither the group nor Trump has indicated what specifically was responsible for the block, the Washington Examiner reported.

In an email on Tuesday, the organization asked for $3 donations to continue its fight against Trump's "dangerous agenda," according to The Hill.

Trump has often played up his support among veterans, as well as the need to care for their needs, and a recent Pew Research Center survey showed that backing among veterans for the president, at 54 percent, remains much higher than among the average population, which stands at 39 percent.

Will Fischer, who is director of government relations for VoteVets, told Vox that Trump's blocking of the organization shows his lack of character, saying "It's not just being unresponsive to veterans, but about being responsive to any voice of dissent whatsoever."

Fischer added that "When I was in the Marine Corps I learned there were 14 leadership traits, including tact, integrity, courage, decisiveness ­— the leadership required to lead women and men in theater of war and certainly in combat — and Donald Trump literally possesses zero of those leadership traits."